In general, in electronic circuits implementing a fully differential amplifier, a need exists for the fully differential amplifier to control common-mode voltage at its output. A typical approach includes using a common mode feedback (CMFB) amplifier. A CMFB amplifier can provide control of the common mode voltage at different nodes that cannot be stabilized by negative differential feedback, where a reference voltage may be provided that can provide a maximum differential voltage gain and/or maximum output voltage swing. The CMFB amplifier can also provide suppression of common mode components that tend to saturate different stages, by applying common mode negative feedback.
A CMFB amplifier senses the common-mode voltage or VA at the output; compares VA with a reference voltage or Vref, and uses a control voltage Vcntrl to control voltage of an internal biasing node of the CMFB amplifier, and provides feedback regulation. In a typical Miller operational amplifier (Op Amp), the internal biasing node is in the active load of a first stage.
The CMFB amplifier circuit may be realized with continuous-time or switched-capacitor structures. Such structures should have a speed performance comparable to the unity-gain frequency of the differential path, otherwise noise from power supplies could be significantly amplified (i.e., power supply rejection would be too small). In a conventional CMFB amplifier, this is usually difficult, since the CMFB amplifier may rely for stability on the differential path compensation. For example, the CMFB path in a Miller amplifier has an additional pole with respect to the differential one which is located at the control node, and the CMFB bandwidth will necessarily be smaller than the differential bandwidth. Depending on the circuit, the bandwidth may be up to two or three times smaller.
Additionally, in structures that do not rely only on Miller compensation, such as multistage amplifiers without Miller compensation, the conventional CMFB amplifier cannot be fully stabilized using the compensation of the differential path.